In the process of producing oil and gas, it is common practice to treat a hydrocarbon-bearing formation with a pressurized fluid to provide flow channels, thereby fracturing the formation. This treatment usually comprises pumping the fluid down the wellbore to cause fissures or fractures in the subterranean rock strata, facilitating greater production of hydrocarbons.
Fluids also are used to control the undesirable production of sand, thereby facilitating flow of hydrocarbons to the wellbore. Well treatment fluids, particularly those used in fracturing, comprise a water or oil based composition incorporating a thickening agent. In many cases, the thickening agent is a polymeric material.
The thickening agent assists in controlling leak-off of the fluid into the formation, and it aids in the transfer of hydraulic fracturing pressure to the rock surfaces. Primarily, however, the thickening agent facilitates the suspension and transfer into the formation of proppant materials that remain in the formation fracture or sand zone when the hydraulic pressure is released, thereby holding the fracture open in the first instance, or stabilizing the sand in the latter instance.
Polymeric thickening agents useful in such fluids are galactomannan gums such as guar and substituted guars such as hydroxypropyl guar and carboxymethylhydroxypropyl guar. Cellulosic polymers such as hydroxyethyl cellulose may be employed, as well as synthetic polymers such as polyacrylamide. Crosslinking of the polymeric materials is employed to increase the viscosity and proppant carrying ability of the fracturing fluid, as well as to increase its high temperature stability. Typical crosslinking agents comprise soluble boron, zirconium, and titanium compounds.
Well treatment fluids are prepared on the surface and then pumped through tubing in the wellbore to the hydrocarbon-bearing subterranean formation. High viscosity, thickened fluid is highly desirable underground within the formation in order to transfer hydraulic pressure efficiently to the rock, resulting in efficient energy transfer to form fractures in the oil producing formation. Also, high viscosity of the treatment fluid assists in reducing fluid leak-off into the formation. However, large amounts of energy are required to pump such thickened fluids through the tubing into the formation. To reduce the amount of energy required in such pumping operations, various methods of delaying crosslinking of the fluid have been developed. By delaying crosslinking, the fluid may be pumped while in a relatively "thin" state, and then used for fracturing after it has "thickened". These techniques allow the pumping of a relatively less viscous fluid having relatively low friction pressures within the well tubing. Later, at the appropriate place and time, crosslinking of the fluid is accomplished at a location near or within the formation. In this way, the advantageous properties of thickened crosslinked fluid are available at the rock face, while at the same time conserving energy during pumping operations.
One typical delayed crosslinking well treatment fluid system comprises borate crosslinked galactomannan gums such as guar or hydroxypropyl guar. The galactomannan polymers, which may be provided as a solid or as a suspension in a hydrocarbon, hydrate in neutral or acidic solution to form a gel. Under these conditions, i.e., pH of 7 or lower, no crosslinking of guar or hydroxypropyl guar will occur with borate ion. To effect borate crosslinking of guar and hydroxypropyl guar, the pH must be raised to at least 9.0. The requirement to raise the pH to this alkaline level is used to delay the crosslinking of the galactomannan gums by the borate ion. Thus at least one difficulty in such methods is that the pH must be made alkaline at precisely the correct stage of the pumping operation to effect crosslinking at the appropriate time.
The practice of delaying crosslinking of thickening agents in such fluids, however, presents other difficulties. Sophisticated techniques must be employed to adjust the pH of the fluid at the proper location, i.e., in or near the formation. U.S. Pat. No. 5,259,455, for example, describes the practice of controlled dissolution of MgO in a fracturing fluid to provide such pH adjustment. Operating effectively where formation temperatures are above 200.degree. F., the patent discloses additives to prevent the magnesium precipitation which would lower the pH of the system. The problems with such systems include the fact that at high use concentrations other precipates are formed which may reduce fracture conductivity.
An alternative approach to the problem of adequately and in a timely manner accomplishing downhole pH adjustment is to reduce the concentration of the galactomannan thickening agent in the well treatment fluid. At such reduced concentrations, also known as "reduced loadings", it is sometimes possible to accomplish crosslinking earlier, or with only a slight amount of delay. In some cases, the reduced loading reduces the friction of pumping the fluid, making the delay of crosslinking less of a significant factor than it might otherwise be in such pumping operations.
Reduction of the galactomannan thickening agent concentration (i.e., use of a low polymer loading) in such fluids, however, has not been practiced to any significant extent because of a long-established belief by those skilled in the art that certain threshold minimum levels of polymer loading of the thickening agents are required for effective or sufficient crosslinking. Conventional wisdom has dictated that the subterranean formation will not adequately fracture or fissure unless a certain minimum threshold of polymer is available to crosslink within the fracture. Also, in the past it has been believed that certain levels of polymer were necessary to provide the viscosity necessary to carry the proppant into the fracture.
In the case of the polymer guar, for example, this threshold concentration has been considered to be about 17 pounds of guar per one thousand gallons of aqueous fracturing fluid. This belief in the art was based upon studies of the radius of gyration of the guar molecule and the theory that if the radius of gyration of two molecules in solution do not overlap, the molecules cannot provide crosslinking in sufficiently great numbers to produce an adequately crosslinked gel as required for reliable fracturing operations.
As a general proposition, well treatment solutions employed in the field utilizing crosslinking of the thickening agent, prior to this invention, customarily utilized concentrations of delayed crosslinking thickening agents that are well above the level previously mentioned-typically in the range of at least 30 pounds of polymer per 1000 gallons of liquid.
Furthermore, other prior art fracturing methods have employed relatively simple buffers to delay crosslinking by producing an acidic pH in the fluid. Acidic pH is required in these prior art methods because it is necessary to maintain such an acidic pH to facilitate the dissolution of the relatively larger concentration of galactomannan gum in the fluid. Such systems typically have been employed at the relatively large polymer loadings mentioned previously. At such loadings, acidic pH is employed in the fluid at the surface (prior to pumping downhole) to solubilize the galactomannan gum.
It has been necessary in the past using certain prior art systems to employ both: (1) a buffer to produce acidic pH, and (2) a delayed action basic compound which acts override the effect of the buffer during pumping, allowing for a gradual rise in pH from acidic levels (below 7) to basic or alkaline conditions (above 7) during pumping of the fluid downhole into the subterranean formation. In this way, slow dissolution of a delayed action compound was used to regulate crosslinking of polymer in such systems.
The disadvantages of using only the slow dissolution method during pumping downhole to regulate crosslinking in such systems are numerous. For example, precise regulation of the pH downhole using such systems is not possible. The adjustment of the pH by slowly soluble sources of delayed action basic compounds is not precise, and errors in pH adjustment are common.
There is not a satisfactory method to monitor and adjust the pH once the fluid is pumped downhole, and in such systems, the passage of time is the controlling factor regulating the dissolution of the delayed action basic compound. Time is a variable that cannot be independently controlled and regulated from the surface, and the operator of such fracturing systems cannot readily change the rate of dissolution of the delayed action basic compound once the fluid is pumped downhole. Thus, slow dissolution of delayed action basic compounds may pose pH problems when used as a method of regulating crosslinking.
Furthermore, soluble borate sources tend to limit the maximum operating temperature of fluids because syneresis of such fluids occurs at low temperatures where pH is greatest. Conventional borate salts and similar compounds are limited as to the maximum temperature that can be achieved. There has been a recognized need for a borate source that will facilitate a longer delay time, thereby allowing pumping into deeper and hotter wells. The fracturing industry needs a crosslinking system that can overcome the inherent limitations of borate sources that limit the maximum operating temperature of the fluid.
A need has existed for a borate source that minimizes the available boron concentration in solution while the fluid is at the surface, thereby facilitating a higher total concentration of boron source in the fluid. In such systems, sufficient amounts of the borate are masked such that most of the borate only becomes available for crosslinking at the higher temperatures found downhole. Unfortunately, soluble borate sources in such fluid systems are limited to maximum temperatures above which they cannot operate effectively. This maximum temperature has been in the range of about 175-200.degree. F.
What has been needed in the industry is a fracturing fluid that does not exhibit syneresis at the surface, but still provides good viscosity at temperatures up to 250.degree. F., or even higher.
It would be desirable to provide a well treatment fluid, especially a fracturing fluid, that exhibits relatively low friction loss in the well tubing, while avoiding the difficulties associated with raising the pH at the proper time or downhole location. Additionally, there has been a search for a system that further avoids difficulties associated with insufficient crosslinking of the polymer at low levels of polymer loading.
There has existed a need in this industry for an effective fluid having reduced concentrations of galactomannan gum polymer, thereby reducing the costs of such solutions and improving the conductivity of oil and gas produced from the formations after fracture treatment. It has been recognized as desirable to devise a method characterized by use of a low cost fracturing fluid that is not dependent on precision pH adjustment downhole, wherein the fluid uses reduced polymer loadings while still sufficiently crosslinking the polymer to adequately fracture the formation, especially at high temperatures. In particular, such a system that also incorporates an improved borate dissolution would be desirable. This invention is designed to meet these and other needs as set forth below.